This invention relates to an optical device for variable attenuation of the intensity of light input thereto.
Known variable optical attenuator comprises a Faraday rotator placed between two polarizers. For example, the optical attenuator of this type is disclosed in JP-B H04-2934. Normally, garnet crystal is used as the Faraday rotator. However, garnet crystal has a property of large wavelength dependence so that the optical attenuator also has the large wavelength dependence of light intensity attenuation function.
Another variable optical attenuator disclosed in JP-A 2000-131626 solves the above-mentioned wavelength dependence problem. The variable optical attenuator of JP-A 2000-131626 comprises incoming and outgoing fibers, a mirror, and a micromachine on which the mirror is movably disposed. A light beam propagated through the incoming fiber is reflected by the mirror. Then, at least one part of the reflected light beam is propagated into the outgoing fiber. The intensity of the light beam within the outgoing fiber depends on the angle of reflection at the mirror, wherein the angle can be adjusted by the rotation of the mirror by means of the micromachine. The micromachine is a silicon micromachine and works in accordance with an electrostatic principle.
The attenuator of JP-A 2000-131626 has some problems. One problem is that it is difficult to produce them at low cost. Another problem is that the attenuation amount of light intensity cannot be adjusted in linearly response to the change of the driving power supplied to the micromachine because of its electrostatic operation principle. This means that it is difficult to adjust the attenuation amount.